Since the pioneering work of Otto Warburg in 1919 (Warburg, 1919), it has been known that cyanide-resistant respiration differentiates most plants and micro-organisms from mammals and other higher animals. Cyanogenic compounds are thus among the most frequently encountered poisons in nature to resist animal predators (Tattersall et al., 2001). Plants and microorganisms are endowed with various components conferring cyanide-resistance, including an unusual, cyanide-resistant mode of respiration. This alternative respiration generally relies on the presence of a unique protein, the so-called alternative oxidase (AOX), which conveys electrons directly from the quinone pool of the mitochondrial respiratory chain (RC) to oxygen, hence by-passing entirely the cytochrome segment of the chain (FIG. 1A) (Affourtit et al., 2002), thereby strongly diminishing proton extrusion linked to substrate oxidation, concomitantly decreasing ATP production. In plants, it therefore prevents the repression of mitochondrial substrate oxidation by high ATP levels resulting from the phosphorylating activity of chloroplasts (Rustin, 1985).
In addition, AOX is considered to act as an antioxidant protein by preventing over-reduction of the mitochondrial quinone pool, which is known to favour superoxide production (Lam et al., 2001; Maxwell et al., 1999). In plants, any significant involvement of the AOX protein in electron flow is triggered only by very peculiar conditions. First, it requires a pronounced reduction of the quinone pool due to the low affinity of the AOX for its quinol substrate (Bahr and Bonner, 1973), and second, the presence of a subset of organic acids, chiefly pyruvate, which regulate the enzyme allosterically (Umbach et al., 2002). Reduced redox status of the RC and a high pyruvate level are the exact conditions resulting from inherited human metabolic disorders localized to the cytochrome segment of the mitochondrial RC (Munnich, 2001). Based on this observation, it has been a long-standing goal of the inventors to express AOX in human cells, with the aim of achieving a potential rescue of electron flow and mitigating the deleterious consequences of pathological RC deficiency. The first attempts to express plant AOX genes in human cells led to apparently uncontrolled lethality (P. Rustin., unpublished data). Even if a genome database search by Vanlerberghe and colleagues revealed the occurrence of AOX in several animal phyla (McDonald and Vanlerberghe, 2004), the goal was not achieved and thus, the solution to the problem of expressing the AOX genes in human cells still remains unsolved. More generally, a need exists for new materials and methods for treating or mitigating the effects of a variety of diseases and conditions related to RC deficiency.